JP5453508B2 - Isolated flyback converter with efficient light load operation - Google Patents

Isolated flyback converter with efficient light load operation Download PDF

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JP5453508B2
JP5453508B2 JP2012254113A JP2012254113A JP5453508B2 JP 5453508 B2 JP5453508 B2 JP 5453508B2 JP 2012254113 A JP2012254113 A JP 2012254113A JP 2012254113 A JP2012254113 A JP 2012254113A JP 5453508 B2 JP5453508 B2 JP 5453508B2
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transistor
current
converter
voltage
time interval
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JP2013158231A5 (en
JP2013158231A (en
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ディー. モリス ジョン
ジー. ネグレ マイケル
ミン チェン
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リニアー テクノロジー コーポレイションLinear Technology Corporation
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    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02MAPPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00Details of apparatus for conversion
    • H02M1/32Means for protecting converters other than automatic disconnection
    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02MAPPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M3/00Conversion of dc power input into dc power output
    • H02M3/22Conversion of dc power input into dc power output with intermediate conversion into ac
    • H02M3/24Conversion of dc power input into dc power output with intermediate conversion into ac by static converters
    • H02M3/28Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac
    • H02M3/325Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal
    • H02M3/335Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/33569Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements
    • H02M3/33576Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements having at least one active switching element at the secondary side of an isolation transformer
    • H02M3/33584Bidirectional converters
    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02MAPPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M3/00Conversion of dc power input into dc power output
    • H02M3/22Conversion of dc power input into dc power output with intermediate conversion into ac
    • H02M3/24Conversion of dc power input into dc power output with intermediate conversion into ac by static converters
    • H02M3/28Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac
    • H02M3/325Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal
    • H02M3/335Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/33569Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements
    • H02M3/33576Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements having at least one active switching element at the secondary side of an isolation transformer
    • H02M3/33592Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements having at least one active switching element at the secondary side of an isolation transformer having a synchronous rectifier circuit or a synchronous freewheeling circuit at the secondary side of an isolation transformer
    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02MAPPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00Details of apparatus for conversion
    • H02M2001/0003Details of control, feedback and regulation circuits
    • H02M2001/0032Control circuits allowing low power mode operation, e.g. "standby"
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02BCLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B70/00Technologies for an efficient end-user side electric power management and consumption
    • Y02B70/10Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion
    • Y02B70/14Reduction of losses in power supplies
    • Y02B70/1458Synchronous rectification
    • Y02B70/1475Synchronous rectification in galvanically isolated DC/DC converters
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02BCLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B70/00Technologies for an efficient end-user side electric power management and consumption
    • Y02B70/10Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion
    • Y02B70/16Efficient standby or energy saving modes, e.g. detecting absence of load or auto-off

Description

  The present invention relates to a DC-DC flyback converter using a synchronous rectifier, and more particularly to such a flyback converter that uses primary side sensing to detect an output voltage.

(background)
DC-DC flyback converters using synchronous rectifiers are well known. If isolation between the input and output stages is required, the output voltage can be sensed by various methods for regulated feedback. Some methods for carrying the output voltage while maintaining isolation include using an optocoupler or using a tertiary winding on the primary side of the transformer. However, these methods require additional circuitry, space, power, and cost. A more elegant way of detecting the output voltage is to sense the voltage at the terminals of the power switch when the power switch is turned off during the discharge (or flyback) cycle of the converter. Such a sensed voltage is substantially proportional to the output voltage. However, since current flows in the secondary winding to generate the primary sense voltage, this scheme requires a minimum duty cycle to be accurate to sense. This scheme also requires a minimum load, generally in the form of a load resistor, to draw a minimum current during the discharge cycle when the actual load is in standby mode with little or no current draw.

  If there is no minimum load resistor and the actual load enters a very low current standby mode, the minimum duty cycle may be greater than the duty cycle required to achieve the regulated output voltage and the output The voltage can exceed the desired regulated level. Therefore, the minimum load current must be above the threshold current to prevent this. Minimum load reduces converter efficiency.

  FIG. 1 illustrates one type of flyback converter 10 that uses a minimum load, when the power switch MOSFET M1 is turned off during a discharge (or flyback) cycle, the flyback converter is in the primary winding. The output voltage VOUT is detected by detecting the voltage. Neither the optocoupler nor the tertiary winding is used to detect VOUT.

  The transformer 12 has a primary winding L1 and a secondary winding L2. MOSFET M1 is controlled by output regulation and control circuit 14 to connect winding L1 between input voltage VIN (eg, battery voltage) and ground during charging.

  In order to achieve regulated VOUT, MOSFET M1 is turned off after a controlled time and synchronous rectifier MOSFET M2 is turned on. The current through winding L2 is transferred at the required voltage to the load and capacitor C1 to be leveled.

For regulated feedback, circuit 14 detects the voltage at the drain of MOSFET M1 during the discharge cycle (MOSFET M1 is off). Sensing the output voltage with a signal on the primary side of the transformer is sometimes referred to as primary side sensing. The drain voltage is related to the winding ratio of L1 and L2, and the voltage of winding L2 is the sum of the output voltage VOUT and the voltage drop of MOSFET M2 (assuming MOSFET M2 is on). . The user selects the value of feedback resistor RFB and the value of reference resistor RREF so that (RFB / RREF) * Vref is equal to the desired regulated voltage (where Vref is internal This is the internal bandgap reference voltage applied to the error amplifier). Such primary side sensing circuits for detecting VOUT are well known and need not be described in detail. All data sheets for the Linear Technology LT3573 flyback converter are incorporated herein by reference and are available online, which describe the operation of the feedback circuit. This operation is also described in US Pat. Nos. 7,471,522 and 7,463,497, which are assigned to the present assignee and incorporated herein by reference. . Other known primary side voltage sensing techniques can be used.

  Circuit 14 continues to control the duty cycle of MOSFET M1 at a variable or fixed frequency to regulate VOUT based on the sensed voltage.

  The circuit 14 can also directly control the synchronous rectifier MOSFET M2 to be turned on when the MOSFET M1 is turned off, or the automatic synchronous switch control circuit 16 can be turned on at an appropriate time. M2 can be controlled. MOSFETs M1 and M2 generally never turn on at the same time. Diode D2 represents the drain-body diode of MOSFET M2.

  The output regulation and control circuit 14 may use any type of conventional technique for regulating, including current mode, voltage mode, or other modes.

  When the load is above a certain threshold current, the conventional operation of converter 10 is used to accurately adjust VOUT. However, if the actual load drops below the threshold current, the required minimum duty cycle of converter 10 will generate too much current and raise VOUT above the regulated voltage. Such light load operation still requires a minimum duty cycle to sample the output voltage on the primary winding L1. In the case where the actual load is of the type having a standby mode that draws very little power, the converter 10 will dissipate the winding L2 current so that regulation can be maintained during periodic cycling of the MOSFETs M1 and M2. To help, a minimum load current resistor R1 is provided. Alternatively or in conjunction, Zener diode D3 is used to ensure that VOUT does not rise above the threshold level. Resistor R1 and zener diode D3 are optional since the minimum current drawn by the actual load may be sufficient to substantially maintain regulation at the lightest load current.

  FIG. 2 illustrates the current through the primary winding L1, the current through the secondary winding L2, and the voltage VM1 of MOSFET M1 for relatively low duty cycle operation. The actual load current can be assumed to be below the minimum current set by the minimum current load resistor R1.

  At time T1, MOSFET M1 is turned on to charge primary winding L1, causing a ramping current to flow in winding L1. MOSFET M2 is off at this time.

  After a variable or fixed time (at time T2), MOSFET M1 stops and MOSFET M2 turns on. This is possible at the minimum duty cycle. This stops the current in the primary winding L1 and reduces the current through the secondary winding L2, while charging the output capacitor C1 and providing current to the load. The voltage on MOSFET M1 is related to the output voltage VOUT and is sampled by circuit 14 during this time. During this light load situation, the current supplied to the capacitor C1 can exceed the avalanche voltage of the Zener diode D3 to increase VOUT and fix VOUT to that value.

  After time T3, the current in the secondary winding L2 decreases to 0 and the MOSFET M2 is turned off to cause the intermittent mode. MOSFET M2 may be turned off by a circuit that detects a slight current reversal through winding L2 by detecting the voltage of MOSFET M2.

  At time T3, the parasitic capacitance of MOSFET M1 and the inductance of winding L1 create a resonant tank circuit.

  At time T4, MOSFET M1 is turned on again and the cycle, which may be the minimum duty cycle, repeats.

  Further details of various converter circuits are described in US Pat. Nos. 5,481,178, 6,127,815, 6,304,066, and 6,307,356. The above references are assigned to the assignee and are hereby incorporated by reference.

  There may be no intermittent operation during the mid-high current mode of converter 10, and converter 10 may operate at a fixed frequency using a variable duty cycle to adjust the output voltage. Such an operation may be conventional.

  During a light load situation (eg, standby mode) of the load, it is important for the converter 10 to draw as little current as possible to maximize battery life. Such a standby mode generally occurs for a relatively long period. It is desirable not to require a minimum current load (eg, resistor R1) to cause converter 10 to adjust VOUT when the actual load is in its standby mode. By removing the minimum current circuit while still achieving substantial regulation when the actual load draws zero or very small current, efficiency is improved and battery life is increased.

(wrap up)
A flyback converter is disclosed that uses primary side sensing to sense the output voltage VOUT, but prevents the output voltage from increasing substantially beyond regulation during light load conditions. Does not require a minimum load current resistor or zener diode. The converter may use any technique (eg, current mode or voltage mode) for adjusting the output voltage during high-intermediate load current.

  During light load current, when the converter operates in intermittent mode (synchronous rectifier is off) and at the same time at the minimum duty cycle, the output voltage is detected on the secondary side of the transformer and the output voltage is adjusted It is compared with a threshold voltage to determine if the voltage has been exceeded. The output voltage can be detected directly at the output terminal of the converter or a resistor divider can be used. Once it is determined that the output voltage has exceeded the threshold, the synchronous rectifier discharges the output capacitor slightly to draw the reverse current through the secondary winding to reduce the output voltage to a nearly regulated voltage. Turned on in a short time. Next, when the synchronous rectifier is turned off, the energy stored in the transformer causes a current to decrease through the drain-body diode of the power MOSFET (power MOSFET is off) in the primary winding. Thus, excess energy is not wasted but reused in the power source (eg, battery). In other words, excess power is transferred from the output side of the converter to the input side. Thus, no minimum load current resistor or zener diode is required, and the converter is much more efficient than the prior art converter of FIG. 1 at light load currents.

  To control the regulation, the timer was off for a sufficient time before the synchronous rectifier was cycled back on again to ensure that there was enough time for the one-side sensing to occur Can be used to detect that.

  In one embodiment, the synchronous rectifier is turned on long enough to reduce the output voltage below the threshold. In another embodiment, the synchronous rectifier can be cycled on and off multiple times to reduce ripple when the output voltage remains above the threshold.

  At the beginning of the next converter switching cycle, the power switch is turned on at the minimum duty cycle to charge the primary winding, and the cycle repeats until the load goes out of its standby mode. Thereafter, the converter operates normally.

  The present invention is used in connection with all of the primary side sensing circuits and can be used with any suitable mode of operation (eg, current mode, voltage mode, burster mode, etc.).

  Although the disclosed embodiments utilize primary side sensing by sensing the voltage at the drain of the MOSFET switch, the primary side sensing can also be by sensing the current in the auxiliary winding on the input side, where Thus, the voltage is related to the voltage of the secondary winding.

For example, the present invention provides the following items.
(Item 1)
A method of operating a flyback converter having a low current load, the converter comprising a transformer having a primary winding and a secondary winding, the primary winding comprising: a power source; a first transistor; The first transistor conducts current through the primary winding when the first transistor is on, and the secondary winding is coupled to the second transistor. The second transistor conducts current through the secondary winding when the second transistor is on, and the converter uses primary-side sensing to cyclically convert the output voltage of the converter The converter has an output capacitor, and the method includes:
Turning on the first transistor for a first time interval at the minimum duty cycle to draw current through the primary winding;
Turning on the second transistor after the first transistor is turned off to draw current through the secondary winding to charge the output capacitor;
Determining whether the output voltage has exceeded a predetermined regulated voltage by a threshold to detect an overvoltage condition resulting from the low current load;
When the overvoltage condition is detected, the second transistor is turned on for a second time interval to conduct reverse current through the secondary winding to reduce the output voltage. And
After the second time interval, turning off the second transistor stops current flow in the secondary winding and current flows in the primary winding and into the power supply And so that excess power is transferred from the secondary side of the transformer to the primary side of the transformer to reduce the overvoltage during low load current conditions. ,Method.
(Item 2)
The method according to the above item, wherein the second time interval is a predetermined fixed time interval.
(Item 3)
The method according to any of the preceding items, wherein the second time interval is a variable time interval required to reduce the output voltage below the threshold.
(Item 4)
A method according to any of the preceding items, wherein the second transistor is turned on and off multiple times before the first transistor is turned on.
(Item 5)
Sensing a voltage representative of the output voltage on a primary side of the transformer to provide a feedback signal for adjusting the output voltage;
Controlling the duty cycle of the first transistor for a load that draws a current greater than the low load current to maintain the output voltage at a regulated voltage;
Switching the first transistor at the minimum duty cycle for a load that draws current below the low load current.
(Item 6)
The first transistor is a first MOSFET having a drain-body diode, and when the second transistor is turned off after the second time interval, the drain-body diode is A method according to any of the preceding items, wherein current is conducted through the winding.
(Item 7)
A method according to any of the preceding items, wherein the primary side sensing comprises sensing a voltage at an end of the primary winding.
(Item 8)
The step of stopping the current flow in the secondary winding by turning off the second transistor and allowing the current to flow in the primary winding is to turn on the first transistor. A method according to any of the preceding items, occurring without.
(Item 9)
After the step of turning on the second transistor after the first transistor is turned off to draw current through the secondary winding to charge the output capacitor, the method is intermittent A method according to any of the preceding items, further comprising turning off the second transistor when the current through the secondary winding drops to approximately zero to cause a mode.
(Item 10)
Sensing that the second transistor has been turned off for a predetermined period of time before allowing the second transistor to be turned on for the second time interval. The method according to any of the preceding items, further comprising:
(Item 11)
Any of the preceding items, wherein the second time interval occurs immediately after the current through the secondary winding drops to approximately zero so that there is no intermittent mode until after the second time interval. the method of.
(Item 12)
A flyback converter, the flyback converter comprising:
A transformer having a primary winding and a secondary winding, the primary winding being coupled to a power source;
A first transistor coupled to the primary winding, wherein the first transistor conducts current through the primary winding when the first transistor is on; and ,
A second transistor, wherein the second transistor conducts current through the secondary winding when the second transistor is on; and
A regulator coupled to the first transistor, the regulator controlling a duty cycle of the first transistor to regulate an output voltage of the converter, the regulator comprising the first transistor; A regulator configured to control the transistors to have a minimum duty cycle;
An output voltage sensor circuit coupled to the transformer, wherein the output voltage sensor circuit senses the output voltage of the converter using primary side sensing;
An output capacitor coupled to the output terminal of the converter;
A synchronous rectifier controller coupled to the second transistor, the synchronous rectifier controller controlling the second transistor to be on or off;
A comparator, connected to a reference voltage indicative of a threshold voltage exceeding the regulated voltage of the converter, and an input coupled to receive a voltage corresponding to the output voltage of the converter; A comparator, wherein the comparator trigger represents an overvoltage condition; and
An output of the comparator coupled to control the synchronous rectifier controller, wherein the synchronous rectifier controller is configured to reduce the overvoltage condition when the overvoltage condition is detected. An output of the comparator to turn on the second transistor for a time interval conducting reverse current through the secondary winding to reduce the output voltage;
A diode coupled to the primary winding, the diode conducting current through the primary winding after the time interval without turning on the first transistor, thereby A diode, wherein power is transferred from the secondary side of the transformer to the power source while mitigating voltage conditions.
(Item 13)
The converter according to any one of the preceding items, wherein the first transistor is a MOSFET, and the diode is a drain-body diode of the MOSFET.
(Item 14)
The converter according to any of the preceding items, further comprising a logic circuit coupled between the comparator and the synchronous rectifier controller.
(Item 15)
A timer circuit for detecting that the second transistor is off with respect to the threshold period, and detecting that the second transistor is off with respect to the threshold period; The converter according to any of the preceding items, wherein the timer circuit controls the logic circuit to turn on the second transistor for the time interval during the overvoltage condition.
(Item 16)
The converter according to any of the preceding items, wherein the time interval that is on so that the second transistor conducts the reverse current is a fixed time interval.
(Item 17)
Any of the preceding items, wherein the time interval when the second transistor is on to conduct the reverse current is a variable time interval required to reduce the output voltage below the threshold voltage. The converter described in Crab.
(Item 18)
The converter according to any of the preceding items, wherein the primary side sensing detects a voltage at a node between the primary winding and the first transistor.
(Item 19)
The converter of any of the preceding items, wherein the regulator is configured to control the first transistor at the minimum duty cycle during the overvoltage condition.
(Item 20)
The synchronous rectifier is also configured to turn off the second transistor at about that time when the current through the secondary winding is zero to generate an intermittent mode of the converter, The output of the comparator is coupled to control the synchronous rectifier controller, and the synchronous rectifier controller is configured to reduce the output voltage of the converter to reduce the overvoltage condition. The converter according to any one of the preceding items, wherein the second transistor is turned on for a period after the mode and the reverse current is conducted through the secondary winding.

(Summary)
The flyback converter uses primary side sensing to sense the output voltage for regulation feedback. Such sensing requires a predetermined minimum duty cycle even with very light load current. Thus, such a minimum duty cycle can generate an overvoltage condition. In the flyback phase, after the minimum duty cycle of the power switch at light load current, the synchronous rectifier is turned off at about the time when the current through the secondary winding drops to zero to produce an intermittent mode. If detected as an overvoltage, the synchronous rectifier is turned on for a short time interval to draw a reverse current through the secondary winding. When the synchronous rectifier stops, current flows through the primary winding through the drain-body diode while the power switch is off. Therefore, excess power is transferred from the secondary side to the power supply to reduce the overvoltage, and therefore excess power is not wasted.

FIG. 1 illustrates a prior art flyback converter. FIG. 2 illustrates not only the current through the transformer winding of FIG. 1 but also the voltage of the power switch when the converter provides a light load current. FIG. 3 illustrates a flyback converter that utilizes the present invention to cycle through a synchronous rectifier to prevent an overvoltage condition when the converter provides light load current or does not provide load current. FIG. 4 illustrates not only the current through the transformer winding of FIG. 3 but also the voltage of the power switch when the converter provides light load current or does not provide load current. FIG. 5 is a flow chart identifying various events that occur during use of the present invention.

  Components that are the same or equivalent are labeled with the same number.

  FIG. 3 represents any of the many types of flyback converters using primary side detection of the output voltage VOUT. Since the present invention is only concerned with the operation of the converter during light load current conditions, any conventional aspect of a flyback converter may be an intermediate-to-high load when the converter operates in intermittent mode and overvoltage occurs. Can be used for current. Such conventional circuitry is well known and there are various types (eg, current mode, voltage mode, variable frequency, fixed frequency, etc.), so there is no need to describe such conventional circuitry in detail. . The description of the conventional aspect of the converter 10 of FIG. 1 is applied to the converter 20 of FIG.

  For intermediate-high load current operation, converter 20 periodically turns on MOSFET M1 to charge primary winding L1. The turn-on time of MOSFET M1 depends on the feedback voltage at the drain of MOSFET M1 associated with VOUT, which was the synchronous rectifier MOSFET M2 on and current was flowing through the secondary winding L2. Sampled in time. The feedback voltage is used to generate a value that is compared to the reference voltage by the error amplifier using resistors RFB and RREF. The error signal generated by the error amplifier sets the time that MOSFET M1 is on during the cycle (ie, sets the duty cycle). This can be conventional.

  In one embodiment, converter 20 is of the voltage mode type, where output regulation and control circuit 14 compares the error signal with a sawtooth waveform. When they cross, for a mid-high current load, MOSFET M1 is turned off to establish a duty cycle that is required to accurately regulate the voltage.

  When converter 20 is of the current mode type, MOSFET M1 remains on until the ramping current signal through MOSFET M1 crosses the error signal.

  The regulation may use any other type of primary side sensing, including using an auxiliary winding on the input side to detect the output voltage.

  MOSFET M2 is turned on when MOSFET M1 is turned off. Many conventional techniques can be used to sense when to turn on MOSFET M2. In one embodiment, synchronous switch control 24 detects the voltage on MOSFET M2. When MOSFET M1 is switched off, the voltage on MOSFET M2 becomes negative (drain voltage is below ground) and this sensed voltage reversal causes synchronous switch control circuit 24 to turn on MOSFET M2. . When the secondary winding L2 current decreases to 0, the drain voltage increases, causing the synchronous switch control circuit 24 to turn off the MOSFET M2. With each cycle when MOSFETs M1 and M2 are turned on and off, a current pulse is provided to the output, and the output is flattened by capacitor C1 to produce a DC regulated output voltage VOUT.

  Various other conventional schemes may also be used to control MOSFET M2 turning on and off to emulate a diode.

  The adjustment scheme may be a variable frequency type or a fixed frequency type.

  FIG. 5 is a flowchart illustrating the various steps performed by the converter 20 in the light load, minimum duty cycle mode, which will be referred to in the following description.

  For primary side sensing, the MOSFET must trigger to generate a voltage on the primary winding L1 in order to detect VOUT. At light loads, very little current may be drawn or no current may be drawn, yet converter 20 may still perform a periodic minimum duty cycle to detect VOUT (FIG. 5). Step 30). A light load can be caused by the load entering standby mode (step 32 of FIG. 5). In this case, the minimum duty cycle is too large for the requested load current and VOUT rises above the desired adjusted value (steps 34 and 36 in FIG. 5).

  FIG. 4 illustrates not only the current in primary winding L1 and secondary winding L2, but also the voltage of MOSFET M1 during light load conditions according to the present invention.

  At time T1, MOSFET M1 is turned on and MOSFET M1 may be under control of a clock for fixed frequency type operation. This produces a ramping current that flows through the primary winding L1.

  After a minimum time (relative to the minimum duty cycle), at time T2, MOSFET M1 is turned off. Such a minimum time is set by a timer of the output adjustment and control circuit 14, which prevents the MOSFET M1 from being turned off before a predetermined minimum time. Such circuitry is conventional.

  At time T2, the synchronous switch control circuit 24 detects inversion of the voltage of the secondary winding L2, and turns on the MOSFET M2. This creates a decreasing current through the secondary winding L2, which charges the capacitor C1 above the desired regulated VOUT level for light load requirements.

  At time T3, the secondary winding L2 current decreases to zero. The synchronous switch control circuit 24 detects a slight increase in the drain voltage, turns off the MOSFET M2, and generates an intermittent mode (step 40 in FIG. 5). If MOSFET M2 was not turned off, reverse current would have flowed through secondary winding L2. The conventional circuit can be used to detect the occurrence of reversal of current in the secondary winding L2 and to switch off the MOSFET M2, which is just before or just after the actual current reversal of the secondary winding L2. Can occur.

  Between times T2 and T3, VOUT can be sampled by the output regulation and control circuit 14 to determine the duty cycle of MOSFET M1 during the next cycle. Although not necessarily required, it is conventional to sample to occur at about that time when the current through the secondary winding L2 is zero. During light load current, the duty cycle is a predetermined minimum duty cycle.

  Comparator 42 receives VOUT or a voltage proportional to VOUT (eg, a resistor divider voltage) and compares it to a reference voltage Vref slightly above the desired regulated voltage. Verf may be equal to VOUT × 1.05.

  At the same time, timer 44 detects that MOSFET M2 is off for a minimum amount of time to ensure that VOUT is sampled on the primary side. Since timer 44 may not be necessary in some cases (eg, sampling occurs before the current through secondary winding L2 is zero), it is optional. If an overvoltage is detected and timer 44 detects that MOSFET M2 has been off for a sufficient amount of time (step 46 of FIG. 5), logic circuit 48 at time T4, the secondary winding To conduct reverse current through L2, the synchronous switch control circuit 24 triggers to turn on MOSFET M2 (step 50 of FIG. 5). This time to turn on can be a fixed time or can occur relative to the time to sufficiently reduce VOUT to trigger the comparator 42. If the turn-on time is a fixed time, multiple cycles to turn MOSFET M2 on and off can be used to reduce VOUT to minimize ripple.

  During the time that MOSFET M2 is on (between times T4 and T5), the voltage traverses MOSFET M1 in relation to the voltage of secondary winding L2.

  At time T5, MOSFET M2 is turned off, causing a voltage reversal of primary winding L1. As shown between times T5 and T6, this causes the drain-body diode D1 of MOSFET M1 to conduct and draws current through the primary winding L1 between times T5 and T6 (FIG. 5). Step 52). Such current flows in the battery supply VIN and thus no power is wasted. Thus, excess power is transferred from the secondary side to the primary side to improve the efficiency of the converter 20 at light loads, and no minimum load current resistor or zener diode is required to reduce the overvoltage (FIG. 5 step 54). In some cases, MOSFET M1 may turn on for a time that diode D1 conducts (eg, when a new charging cycle begins in response to a clock pulse).

  During the time when both MOSFETs are off, a tank circuit is created causing the oscillation of MOSFET M1.

  In another embodiment, instead of a drain-body diode D1 conducting current through the primary winding L1 during times T5 to T6, after a reverse current time interval, the voltage variation in the primary winding L1 is sensed. A sensing circuit may be added to turn on MOSFET M1 to conduct excess power into the power supply. Since circuit 14 typically only turns on MOSFET M1 at the start of a clock cycle, control of such MOSFET M1 may be independent of output regulation and control circuit 14. Such a technique may be useful when the power switch does not include an intrinsic diode between the primary winding L1 and ground.

  In yet another embodiment, the comparator 42 detects that the output voltage is greater than the desired regulated voltage and sets the MOSFET M2 on-state as long as the output voltage is required to decrease below Vref. keep. For example, with respect to FIG. 4, at time T3, the synchronous switch control circuit 24, the comparator 42, and the logic 48 do not initially enter the discontinuous mode, but conduct a reverse current through the secondary winding L2, and output voltage To keep the MOSFET M2 in the on-state. Once comparator 42 detects that the output voltage has dropped below Vref, comparator 42 triggers MOSFET M2 to turn off, causing a discontinuous mode. In another embodiment, the intermittent mode can be any period (including zero) after the current through the secondary winding L2 has dropped to zero. The comparator 42 can have hysteresis.

  The present invention may be used during fixed frequency operation of converter 20 or during certain light load modes of operation where MOSFET M1 is not turned on at a fixed frequency.

  The MOSFET can alternatively be a bipolar transistor.

  One skilled in the art can design various functional blocks in many ways without undue experimentation and using conventional circuit techniques.

  While particular embodiments of the present invention have been shown and described, it will be apparent to those skilled in the art that changes and modifications can be made in the broader aspects of the invention without departing from the invention. The appended claims encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention.

Claims (19)

  1. A method of operating a flyback converter having a low current load, the converter comprising a transformer having a primary winding and a secondary winding, the primary winding comprising: a power source; a first transistor; The first transistor conducts current through the primary winding when the first transistor is on, and the secondary winding is coupled to the second transistor. The second transistor conducts current through the secondary winding when the second transistor is on, and the converter uses primary-side sensing to cyclically convert the output voltage of the converter to sense the has a duty cycle which is adjusted primary with respect to the load current above a threshold level, and a primary minimum duty cycle for a load current below the threshold value level, the converter An output capacitor, the method comprising,
    Turning on the first transistor for a first time interval at the minimum duty cycle to draw current through the primary winding due to the load current below the threshold level ; ,
    Turning on the second transistor after the first transistor is turned off to draw current through the secondary winding to charge the output capacitor;
    Performing the primary side sensing of the output voltage;
    To detect over-voltage conditions resulting from low current load, and that said output voltage to determine whether more than just a certain threshold a predetermined regulated voltage,
    When the overvoltage condition is detected, the second transistor is turned on for a second time interval to conduct reverse current through the secondary winding to reduce the output voltage. And
    After the second time interval, turning off the second transistor stops current flow in the secondary winding and current flows in the primary winding and into the power supply And so that excess power is transferred from the secondary side of the transformer to the primary side of the transformer to reduce the overvoltage during low load current conditions. ,Method.
  2.   The method of claim 1, wherein the second time interval is a predetermined fixed time interval.
  3.   The method of claim 1, wherein the second time interval is a variable time interval required to reduce the output voltage below the threshold.
  4.   The method of claim 1, wherein the second transistor is turned on and off multiple times before the first transistor is turned on.
  5. Sensing a voltage representative of the output voltage at a primary side of the transformer to provide a feedback signal for adjusting the output voltage;
    And that in order to maintain the output voltage to the regulated voltage, the load pulling the greater current than the low load current, controls the de-menu tee cycle of said first transistor,
    The method of claim 1, further comprising: switching the first transistor at the minimum duty cycle for a load that draws current below the threshold level .
  6.   The first transistor is a first MOSFET having a drain-body diode, and when the second transistor is turned off after the second time interval, the drain-body diode is The method of claim 1, wherein current is conducted through the winding.
  7.   The method of claim 1, wherein the primary side sensing includes sensing a voltage at an end of the primary winding.
  8.   Stopping the current flow in the secondary winding by turning off the second transistor and allowing the current to flow in the primary winding turns on the first transistor. The method of claim 1, occurring without.
  9. After the step of turning on the second transistor after the first transistor is turned off to draw current through the secondary winding to charge the output capacitor, the method is intermittent to cause mode, when the current through the secondary winding is decreased to approximately 0, further seen including turning off the transistors of the second, the second for the second time interval The step of turning on a transistor comprises turning the second transistor back on after the second transistor is turned off and before the first transistor is turned back on. the method of.
  10.   Sensing that the second transistor has been turned off for a predetermined period of time before allowing the second transistor to be turned on for the second time interval. The method of claim 9 further comprising:
  11.   The method of claim 1, wherein the second time interval occurs immediately after the current through the secondary winding has dropped to approximately zero, such that there is no intermittent mode until after the second time interval. .
  12. A flyback converter, the flyback converter comprising:
    A transformer having a primary winding and a secondary winding, the primary winding being coupled to a power source;
    A first transistor coupled to the primary winding, wherein the first transistor conducts current through the primary winding when the first transistor is on; and ,
    A second transistor, wherein the second transistor conducts current through the secondary winding when the second transistor is on; and
    A regulator coupled to the first transistor, the regulator controlling a duty cycle of the first transistor to regulate an output voltage of the converter, the regulator comprising:
    Is configured for the first transistor is controlled to have a duty cycle that is adjusted primary with respect to the load current above a threshold level, and a primary minimum duty cycle for a load current below the threshold value level , The regulator,
    An output voltage sensor circuit coupled to the transformer, wherein the output voltage sensor circuit senses the output voltage of the converter using primary side sensing;
    An output capacitor coupled to the output terminal of the converter;
    A synchronous rectifier controller coupled to the second transistor, the synchronous rectifier controller controlling the second transistor to be on or off;
    A comparator, connected to a reference voltage indicative of a threshold voltage exceeding the regulated voltage of the converter, and an input coupled to receive a voltage corresponding to the output voltage of the converter; A comparator, wherein the comparator trigger represents an overvoltage condition; and
    An output of the comparator coupled to control the synchronous rectifier controller during operation of the regulator having the load current below the threshold level at the minimum duty cycle, rectifier controller, when the over-voltage condition is detected, in order to reduce the output voltage of the converter to reduce the over-voltage state, to the time interval for conducting a reverse current through the secondary winding An output of the comparator to turn on the second transistor;
    A diode coupled to the primary winding, the diode conducting current through the primary winding after the time interval without turning on the first transistor, thereby A diode, wherein power is transferred from the secondary side of the transformer to the power source while mitigating voltage conditions.
  13.   The converter of claim 12, wherein the first transistor is a MOSFET and the diode is a drain-body diode of the MOSFET.
  14.   The converter of claim 12, further comprising a logic circuit coupled between the comparator and the synchronous rectifier controller.
  15.   A timer circuit for detecting that the second transistor is off for a threshold period, and detecting that the second transistor is off for the threshold period; The converter of claim 14, wherein the timer circuit controls the logic circuit to turn on the second transistor for the time interval during the overvoltage condition.
  16.   The converter of claim 12, wherein the time interval that is on so that the second transistor conducts the reverse current is a fixed time interval.
  17.   13. The time interval during which the second transistor is on to conduct the reverse current is a variable time interval required to reduce the output voltage below the threshold voltage. The listed converter.
  18.   The converter of claim 12, wherein the primary side sensing detects a voltage at a node between the primary winding and the first transistor.
  19.   The synchronous rectifier is also configured to turn off the second transistor at about the time when the current through the secondary winding is zero to generate an intermittent mode of the converter, The output of the comparator is coupled to control the synchronous rectifier controller, and the synchronous rectifier controller is configured to reduce the output voltage of the converter to reduce the overvoltage condition. The converter of claim 12, wherein the second transistor is turned on for a period after mode to conduct the reverse current through the secondary winding.
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